Mental Math Basics, what is your most useful tip?

[KMD]

I think as the toolbox leaks out, the politics of DIR will fade. That only occurs when there is a vacume. Very useful stuff.

Cough! Snort!

 

[MXGratefulDiver]

Mental math basics? Here's an easy rule of thumb I give my students ...

"Never go deeper than the cubic feet of gas your tank holds."

In other words ... don't take an AL80 below 80 feet. Don't take a 100 CF tank below 100 feet ... etc.

It's real rough, and doesn't apply so much to more experienced divers ... but it does tend to help keep some less experienced divers outta trouble ...

... Bob (Grateful Diver)

 

[Kevrumbo]

2 bar per minute at 1 ATA; multiply this value by the depth you're at in ATA and you have your pressure consumption rate. Example: 20msw depth translates to 3 ATA. Multiply 2 by 3 ATA and now you have a depth consumption rate of 6 bar/min. So by 10 minutes of easy nominal work at 20msw, you should expect your SPG to have a delta of 60 bar down from the previous reading. . . (or checking more frequently, by 5 minutes you should have a delta of 30 bar down etc.)

Assumptions: 11 Liter Tank (the ubiquitous AL 80 Tank); Nominal SAC rate of 21.1 Liter/min*ATA (Imperial Units: 0.75cft/min*ATA).

[Come over to the real "Dark Side" --use the Metric System! ]

 

[Jasonmh]

Great thread topic Catherine.
I am also pathetically bad at math in my head, something I am trying to work on. I have found some of Peter's advice helpful for doing stuff in my head:

http://dir-diver.com/en/knowledge/calculations_on_the_fly.html

http://dir-diver.com/en/knowledge/dive_planning_on_the_fly.html

GUE's min. deco limit is what some agencies call NDL. Ratio Deco is for dives that exceed min. deco, and there are different types (1:1, 1:2, etc.)

 

[Cacia]

have a delta

the triangle "change"

what is delta?

 

[TSandM]

Delta = triangle = change.

Comes from math -- delta P = change in pressure.

 

[grazie42]

Originally posted by Kevrumbo
Nominal SAC rate of 21.1 Liter/min*ATA

Without starting a pissing contest...isn´t that rather high? My instructor told me that a normal "decent" recdiver will use 15lpm...

P.S Kudos for going metric, sometimes there´s light in the darkness

 

[Kevrumbo]

Without starting a pissing contest...isn´t that rather high? My instructor told me that a normal "decent" recdiver will use 15lpm...

P.S Kudos for going metric, sometimes there´s light in the darkness

The SAC rate is a little high for conservatism, and to make its application for "on-the-fly/in-your-head" calculations easier. So given a SAC rate of 21 Liters/min*ATA:

And for convenience of calculation, say you use a twinset of 11 Liter Bottles. You then would have a metric standard tank factor of 2*11 equals 22 Liters per bar. To figure out your SAC rate now in terms of pressure, you simply divide 21 by 22 Liters per bar . . .and you come out in this instance, nearly 1 bar/min*ATA (acutally rounded up from 0.95 bar/min*ATA).

Now use the 1 bar per minute nominal SAC rate with a twinset of 11 Liter bottles. Since you're using a normalized rate of 1 bar/minute, all the arithmetic you have to do to figure gas consumption on-the-fly at depth simply involves the time and depth you're at in ATA.

Quick example: an easy nominal dive at 30msw which equals 4 ATA, for 20 minutes NDL/MDL. And so then 4*20 translates to 80 bar delta of gas used at this depth.
Or 4*10 translates to 40 bar delta, if you only stay 10 minutes at this depth.
Or 4*5 translates to 20 bar delta if you decide to check your SPG at the 5 minute mark, etc.

(Note: at depths greater than 30msw, you should have an intuitive feel for how much gas you're using every 5 minutes with the above method, and verifying it with a quick glance at the SPG. . .)

 

[ShoalDiverSA]

More metric examples, please!

This is a great thread, thanks. The first step to weaning me off my computer....

Cheers,

Andrew

 

[TheFoggyMask]

Mental math basics? Here's an easy rule of thumb I give my students ...

"Never go deeper than the cubic feet of gas your tank holds."

In other words ... don't take an AL80 below 80 feet. Don't take a 100 CF tank below 100 feet ... etc.

It's real rough, and doesn't apply so much to more experienced divers ... but it does tend to help keep some less experienced divers outta trouble ...

... Bob (Grateful Diver)

Unfortunately, not everyone gets the education they should. Especially when it comes to gas planning.